Bootstrap power supplies are known. Most bootstrap power supply circuits are used in consumer electronic devices. Typical gate driver integrated circuits are designed to function with a bootstrap supply, for example.
FIG. 1 illustrates a typical bootstrap power supply arrangement 10 for providing power to a load 12. A voltage source 14 has an associated current limiting resistor 16 and a rectifier 18. A low side IGBT 20 switches on and off in a known manner. A high side IGBT 22 is controlled by an upper gate driver 24. A bootstrap capacitor 26 powers the upper gate driver 24.
The voltage source 14 charges the bootstrap capacitor 26 when the low side IGBT 20 is conducting. When the low side IGBT 20 is turned off, the bootstrap capacitor 26 is left floating. In that condition, the bootstrap capacitor 26 is connected with the upper gate driver 24 and the emitter of the high side IGBT 22. When the low side IGBT 20 is off, the voltage drop across it effectively changes the voltage that is used for charging the bootstrap capacitor 26. Under those conditions, the power supply for the upper gate driver 24 is unregulated. This is undesirable in many circumstances. For example, the unregulated power supply renders it of limited use for high voltage applications.
In a high voltage arrangement, the voltage drops across the low side IGBT 20 can be significant compared to the voltage source 14. This produces a large variation in the charging voltage of the bootstrap capacitor 26 and makes it very difficult to remain within the desired voltage range for the upper gate driver 24. The large voltage variation also has a negative effect on the switching performance of the IGBTs 20 and 22.
Another limitation on the typical arrangement shown in FIG. 1 is that the rate at which the bootstrap capacitor 26 can be charged is relatively slow. This requires a larger resister 16 in series with the diode 18. The current limiting resistor 16 protects the upper gate driver 24 from a high change rate of voltage at initial turn on and limits the rate at which the bootstrap capacitor 26 can be charged. For some high voltage situations, the amount of time available for charging the bootstrap capacitor 26 is very limited and without a sufficiently high rate of charging, the arrangement shown in FIG. 1 is not useful.